Inhibitory action of alpha-melanocyte stimulating hormone on lordosis in rats: possible involvement of serotonin.

Alpha-melanocyte stimulating hormone (MSH) has been found to exert a short- and a long-term inhibitory action on lordosis. The present series of experiments examined the possibility that these effects are mediated by MSH-induced alterations in activity at serotonin type II receptors. In Experiment 1, quipazine (serotonin type II agonist) was found to significantly attenuate the short-term effect of MSH while only partially attenuating the long-term action of MSH. In the second experiment, doses of MSH and of pirenperone (serotonin type II antagonist) that did not inhibit lordotic responding when administered alone were found to inhibit responding when administered together. It was also found that 20 ng MSH produced a long-term, but not a short-term inhibition of receptivity. The results of Experiment 3 indicated that the inhibition observed in Experiment 2 could be reversed by quipazine. These results suggest that alterations in serotonin activity are one mechanism by which the effects of MSH are produced. The relevance of this to the regulation of reproductive states is discussed.